4. Design a Notification System
Deliver email, push and SMS with preferences, retries and honest delivery status.
ClassicMedium
Guided practice
Start with a template. Work through each step. Ask Coach when you need a second opinion.
Designs with worked examples 23
Ordered by design numberCompany tags are community-reported. Counts on cards show how many people reported that design.
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6. Design a News Feed
Build a paginated feed that handles high-fan-out authors, fresh posts and visibility changes.
ClassicMedium30. Transactional Outbox
A service commits to its database and then publishes an event. If it dies in between, the row exists and no one hears about it. Publishing first has the mirror problem: an event for a write that never happened. There is no ordering of two independent systems that makes this safe.
Product & backendMedium40. Design an Ad Click Aggregator
Count every ad click once, fast enough to chart live and exactly enough to bill. One-hour interview boards for junior, senior and staff: requirements, data layer, low-level design and what goes wrong at every component.
ClassicMedium41. Ad Click Aggregator: Low-Level Design
The classes and code behind the ad click aggregator: signed click tokens, a redirect that never waits on the log, and a stream counter that de-duplicates and handles late clicks. Tests run all of it.
Data engineeringMedium42. Design a Metrics Monitoring Platform
Collect 5 million samples a second from 500,000 hosts, store them as time series, chart them and page people.
ClassicMedium43. Design YouTube Top-K
The K most-viewed videos for the last hour, day, month and all time from 700,000 views a second, exactly and in milliseconds.
ClassicMedium44. Design a Price Tracking Service
Watch prices on 500 million products with a polite crawler and a million browsers, verify what you are told, and notify subscribers within minutes of a drop.
ClassicMedium49. Design an Online Auction like eBay
Take bids on 10 million live auctions with one consistent highest bid, lose none, push the price to every watcher and end each auction fairly.
ClassicMedium52. Design a Payment System like Stripe
Charge cards through the networks exactly once: idempotency keys, a card vault, timeouts as unknowns, signed webhooks, a double-entry ledger and reconciliation. One-hour boards for junior, senior and staff, with the theory behind them.
ClassicMedium57. Design Post Search like Facebook
Search a billion posts a day by keyword, newest or most liked first, with an inverted index you build yourself.
ClassicMedium67. Design for Write Scale: A Million Writes a Second
A live TV vote at a million writes a second: spread by key, buffer in Kafka, aggregate the hot counter in two stages, shed what can wait.
ClassicMedium72. Design a Distributed Log like Kafka
A partitioned, replicated append-only log: a million messages a second, order per key, nothing acknowledged ever lost, a week of replayable history.
ClassicMedium77. Design a Stream Processor like Flink
A stateful stream processor: dataflow graphs, keyed state in RocksDB, event time and watermarks, windows, barrier checkpoints, exactly-once into Kafka.
ClassicMedium85. Design a Change Data Capture Pipeline
Every committed change in Aurora and DynamoDB, read from the log and delivered in order per row to search, caches, Redshift and an S3 lake.
ClassicMedium88. Design a Reviews and Ratings System
Dish reviews only from customers who ordered them, votes counted from a change stream, reviews ranked by the Wilson bound, payouts made exactly once.
ClassicMedium91. Design a CI/CD System like GitHub Actions
Turn 10 pushes a second into job graphs on ephemeral runners, with fair queues per pool, leases fenced by attempt and live logs.
ClassicMedium93. Design a Real-Time Game Leaderboard
Rank 200 million players in real time with sharded Redis sorted sets, count every game exactly once, and close a season fairly.
ClassicMedium94. Design a Driver Heatmap
Count 4 million moving drivers per map cell from a million pings a second, and serve the density to a million viewers as cached map tiles.
ClassicMedium97. Design a Distributed Logging and Error Tracking System
Ten terabytes of logs an hour from host agents through Kafka into tiered OpenSearch and S3, with regex search, live tail and exceptions grouped into issues.
ClassicMedium101. Design a Webhook Delivery System
Signed HTTPS callbacks for a billion events a day, at least once, retried for three days, with no endpoint able to slow another.
ClassicMedium104. Design a Feature Store
Define a feature once, train on point-in-time correct history and serve 200 fresh values in under 10 ms.
ClassicMedium106. Design an LLM Chat Assistant like ChatGPT
A chat assistant on models we train and serve: next-token framing, three training stages, resumable SSE streams, prefix caching, token quotas, safety, evaluation.
ClassicMedium